Oven source for atomic beam tubes having a non-wettably coated gas passageway between the reservoir and the beam



June 11, 1969 R. H. KERN ETAL 3,450,876

OVEN SOURCE FOR ATOMIC BEAM TUBES HAVING A NON-WETTABLY COATED GASPASSAGEWAY BETWEEN.THE 1 RESER IR AND THE BEAM Fil July 11. 1966 9 e r|8-L FREQUENCY MULTIPLIER I I I4) I5 I? l i g REFERENCE i CONTROL 1OSCILLATOR MODULATOR cmcun INVENTORS ROBERT H KERN JOSEP H. H0 LOWAY ATTUnited States Patent and Joseph H. Holloway, Topsby mesne assignments,to Hew- Palo Alto, Calif., a corporation 6 Claims ABSTRACT OF THEDISCLOSURE An atomic beam tube including an oven source having portionsof its interior coated with a dichlorodimethylsilane material which isresistant to wetting and chemical attack by certain atomic beammaterials at their operating temperatures and pressures. Thenon-wettablc coating eliminates creepage and spillage of atomic beammaterial into the tube.

The present invention relates in general to oven beam sources for atomicbeam tubes and, more particularly to an improved oven source whereincertain gas passageway defining members are coated with a material whichis non-wettable by the atomic beam material, whereby undesired spillageof atomic beam material into the tube is prevented by preventingundesired creepage of the atomic beam material in the liquid phasewithin the oven due to capillary attraction and surface wetting effects.Such an improved oven source is especially useful in cesium atomic beamtubes employed as frequency standards, atomic clocks, and the like.

Heretofore, oven sources for atomic beam tubes have employed antispilldevices interconnecting the reservoir of atomic material with theupstream end of the beam collimator. Moreover, the ovens have beendesigned with built-in temperatures gradients to prevent condensation ofthe vaporized atomic beam material in and around the beam collimator. Inspite of these elaborate precautions, it has been observed that acertain amount of spillage of atomic beam material was occurring fromthe oven through the beam collimator and into the tube. The result ofsuch spillage, aside from wasting the beam material and thus reducingthe tubes operating life, was that it produced a large background signalas well as an altered resonance signal. In such a case, the increasedsignal may exceed the dynamic range of the frequency control circuitryof the frequency standard, thus, interrupting operation of the frequencystandard. For many applications of frequency standards suchinterruptions of operation are intolerable.

It has been found that the atomic beam material such as, for example,cesium was wetting the surfaces of the collimator tubes or straws andalso the tubular structure of the internal antispill network. As aresult the atomic beam material, in the liquid phase, was creepingthrough the antispill tubulation into the region of the collimator andthrough the collimator into the tube. A search for suitable metals thatwould not be wet by the atomic beam material did not turn up any suchsuitable metals.

In the present invention, the antispill tubulation and the beamcollimator are coated with a material that is resistant to attack by theatomic beam material under the operating conditions of temperature andpressure, is not wet by the atomic beam material, and which withstandsthe tube processing temperatures of 350 C. for 24 hours under vacuum of10* torr or less. In one preferred embodiment of the present invention,(CH SiCl (Dry Film, a product of General Electric) is used as thecoating material in ovens using cesium beam material. Coating materialsuseful with atomic beam materials other than cesium, such as sodium,potassium, and thallium, are Teflon and Dry Film. A particularlyconvenient combination of collimator and antispill material and coatingmaterial is oxidized stainless steel metal coated with (CH SiCl Thiscoating material readily adheres by chemical bond oxides. Stainlesssteel is readily oxidized during the conventional copper brazing processwherein parts are heated to copper brazing temperature in a moisthydrogen atmosphere. It has been found that in ovens having theantispill tubulation and the beam collimator coated with suchnon-wettable materials, that spillage of atomic beam material into thetube is eliminated.

The principal object of the present invention is the provision of animproved atomic beam oven and atomic beam tubes using same.

One feature of the present invention is the provision of an atomic beamoven having gas passageway defining portions thereof coated with amaterial which is non-wettable and resistant to chemical attack by theatomic beam material, whereby spillage of atomic beam material from theoven into the tube is prevented in use.

Another feature of the present invention is the same as the precedingfeature wherein the coated gas passageway defining portions include thebeam collimator and/or the antispill tubulation.

Another feature of the present invention is the same as any one or moreof the preceeding features wherein the coating material is selected fromthe class of alkylchlorosilanes, long chain saturated hydrocarbon, andTeflon.

Another feature of the present invention is the same as any one or moreof the preceding features wherein the coating material is Dry Film, theatomic beam material is cesium and the material which is coated by theDry Film is oxidized stainless steel.

Other features and advantages of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawings wherein:

FIG. 1 is a schematic diagram of an atomic frequency standard employingfeatures of the present invention,

FIG. 2 is a longitudinal sectional view of an atomic beam oven sourceemploying features of the present invention,

FIG. 2A is a detail of a portion of the structure of FIG. 2 delineatedby line A-A, and

FIG. 3 is a view of a portion of the structure of FIG. 2 taken alongline 33 in the direction of the arrows.

Referring now to FIG. 1 there is shown an atomic beam tube 1. The tube 1includes a vacuum envelope 2 containing an oven type atomic beam source3 disposed at one end of the envelope 2. The oven 3 will be more fullydescribed below and serves to project a stream of collimated atomicparticles, at thermal velocity, over a predetermined beam path 4. Afirst state selecting magnet assembly 5 deflects out of the beam theatomic beam particles of the undesired energy states. The selectedenergy state atoms then pass through a cavity resonator structure 6containing split field interaction regions 7 and 8. A weak magneticfield as of gauss is provided in the cavity region 6 by means of aC-field magnet 9 to separate the magnetic field dependent resonancelines of the atoms from the desired field independent resonance line.Microwave energy at the resonance frequency of the atoms is supplied tothe cavity resonator 6 from a micro wave generator 11 to exciteresonance of the atoms. A second energy state selecting magnet assembly.12 is disposed downstream of the resonance C-field region to defleetbeam particles that have undergone resonance into a target detector 13to produce a resonance output signal. A modulator 14 modulates theresonance conditions in the resonance region as by modulating thefrequency of the applied microwave energy. This produces a modulationcomponent on the output signal which may be phase detected in a controlcircuit 15 to control the carrier frequency of the applied microwaveenergy. The microwave generator 11 includes a reference oscillator 16which provides an output at 17 at some convenient low frequency such asmHz. which output is locked to the resonance line of the atomic beamparticles. Another output of the reference oscillator is used to providethe applied microwave energy by suitable multiplication in frequencymultiplier 18.

Referring now to FIGS. 2 and 3 there is shown an oven structure of thepresent invention. The oven 3 includes a main block body portion 28 asof copper having a reservoir chamber 21 formed in the lower portionthereof and having a collimator chamber 24 formed in the upper half ofthe body 28. A collimator 23 is formed in one wall of the collimatorchamber 24. The collimator 23 is formed by stacked layers of crinkledstainless steel foil as shown in the detail of FIG. 2A. Each layer ofcrinkled stainless steel foil is about 0.004" thick and 0.188" long withthe wavelength for the crinkles being about 0.010" long. The crinkledfoil is sandwiched between a pair of fiat stainless steel foils. Thecrinkled regions define small diameter tubes or straws which are 0.188"long through which the stream of atomic material such as, for example,cesium, thallium, rubidium, sodium or potassium etfuses. In a typicalexample, the cross section of the bundle of straws, which defines thesize of the beam is 0.020" x 0.125". A Monel frame 29 holds thecollimator 23 therein and is brazed into the copper body 28.

A stainless steel ampule 31 is contained in the reservoir chamber 21. Acompression spring 32 holds the ampule 31 against an electrode 33. Apulse of current, when applied across the electrode 33 to ground,produces heating and vaporization of a thin portion 34 of the ampule 31to open the ampule 31 and permit escape of the atomic material storedtherein into the reservoir chamber 21. The sealed ampule 31 permitsbakeout and evacuation of the over to 350 C. for 24 hours duringprocessing of the tube 1 without loss of the atomic material which wouldotherwise be lost.

An antispill tube 35 as of stainless steel projects reentrantly into thereservoir 21. The reentrant length of the antispill tube 35 is selectedsuch that the open inner end of the tube 35 is above the liquid level inthe reservoir for all possible orientations of the oven 3. A pinch offprotector cap 36, carried over the pinched off fill tube 37 of theampule 31, serves as a cover for the open end of the antispill tube 35to prevent splashing of atomic material into the end of the tube 35. Asecond stage antispill tube 38 as of stainless steel is coaxiallydisposed of the first tube 35. The inner end of the pinch off cap 36also serves as a cap for the second antispill tube 38.

A constrictive tube 39 forms the constricted gas passageway 25 betweenthe collimator chamber 24 and the reservoir 21. The constrictive tube 39as of stainless steel, is mounted coaxially of and within the secondstage of the antispill tubes, thus, forming a third stage of theantispill tubular network. In a typical example, the constrictive tube39 is 0.025" inside diameter by 0.064 outside diameter and 0.200" longand provides a conductance of about 3.2)(- liters/second.

The collimator 23 and the reentrant antispill tubulation comprisingtubes 35, 38 and 39 define portions of the gas passageway for the atomicbeam vapor and are all coated with a material which will not be wet bythe liquid atomic beam material, which will resist chemical attack bysuch liquid, and preferably which will withstand the tube processingsteps of bakeout at 350 C. for 24 hours while being pumped by a vacuumpump to 10- torr or less. By coating these parts with a non-wettablecoating, the liquid beam material will not creep through these 'gaspassageways to cause spillage of liquid beam material into the tube. Thecoating destroys any possible capillary tendency for the liquid beammaterial to be drawn into the antispill or collimator tubes.

Non-wettable coating materials include dichlorodimethylsilane having thechemical formula (CH SiCl (Dry Film), Teflon, and long straight chainsaturated hydrocarbons such as Parafiint, a trademarked composition soldby the Moore and Munger Co. of New York and having a chemical formula ofthe form CH (CH ),,CH where n ranges from 40 to 60. However, in apreferred embodiment, the coating material should withstand tube bakeoutat 350 C. at 10- torr for 24 hours. Paraffins are unsuited for thissince they break down at temperatures below 350 C. Also the coatingmaterial should be resistant to chemical attack by the beam material,such as Rb, Cs, K, Na and T1, under operating conditions of temperatureand pressure. In the case of cesium and rubidium, Teflon is attacked byrubidium and cesium under operating temperatures of C. and pressure of10 to l0 torr. However (CH SiCl (Dry Film) is a very satisfactorycoating material for cesium and the other beam materials as it resistschemical attack by these materials and will withstand the bakeoutprocess. Moreover, Dry Film is easily applied to selective regions ofthe oven 3 since it forms a chemical type bond to oxidized surfaces.This is especially convenient since stainless steel is oxidized whereascopper and certain other metals such as Monel are reduced by theconventional copper-silver entectic brazing process wherein parts areheated to brazing temperature in a hydrogen atmosphere moist with watervapor. Thus, by making the parts to be coated with Dry Film out ofstainless steel and then brazing the assembly together the stainlesssteel parts are oxidized and the remaining parts are reduced or cleaned.Then the brazed assembly is heated to about C. and air moist with Watervapor is passed through the assembly for about 1 minute to moisten thesurfaces to be coated with water. The air which is passed through theassembly is then first bubbled through Dry Film liquid to saturate theair with Dry Film. The Dry Film laden air is then passed into theassembly. The Dry Film, in the presence of the prior water coating,reacts with the oxidized Cr O constitutent of the stainless steel toform a chemical bond between the silicon atoms of the Dry Film and theoxygen atoms of the oxidized stainless steel. More particularly, thechlorine atoms of the Dry Film are replaced with the oxygen atoms of thestainless steel in the presence of the water. The assembly is then dryedand is ready to assemble in the tube for subsequent tube processing anduse.

In a preferred embodiment of the oven of the present invention, the galsconductance of the antispill tubulation, including the constrictive tube39, is less than 3 times the conductance of the collimator, whereby thevapor pressure at the upstream end of the collimator 23 is reduced by atleast 25%, and preferably more compared to the vapor pressure inside thereservoir 21. With this arrangement, condensation of vaporized atomicbeam material in the collimator chamber 24 is prevented even though theoven 3 is operated at one temperature throughout or even operated withthe collimator region 24 slightly lower in temperature than thereservoir 21. The oven design having the constriction between thereservoir 21 and the collimator 23 is described and claimed in copendingUS. application Ser. No. 564,215 filed July 11, 1966, and assigned tothe same assignee as the present invention.

Referring now to FIG. 3 the oven 3 includes a mounting face portion 42having a pair of mounting holes 43 therein for receiving studs formounting the oven to a vertical support structure, not shown. A pair ofheater elements are contained in a second pair of bores 44 for heatingthe oven 3 to its operating temperature, as of 5 85 C.:10 C. for cesium.A temperature sensing thermistor assembly 45 is mounted to the flange 42via a spring clip 46 which is held down by a pair of screws 47. Thethermistor assembly 45 serves as a part of a thermal control circuit forholding the temperature of the oven constant as of :01" C.

The oven of FIGS. 3, 4 and 5 when employed in cesium atomic beam tubeshas been successful in preventing condensation of cesium vapor in thecollimator region and in preventing spillage of cesium into the tube.

Since many changes could be made in the above construction and manyapparently widely diflferent embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. An atomic beam oven for atomic beam tubes including means forming areservoir for containing a supply of liquid atomic beam material aportion of which is to 'be vaporized in use to provide a source ofatomic beam vapor material, means defining a gas passageway leading outof said reservoir through which the vaporized atomic beam materialpasses, and means providing a dichlorodimet-hylsilane coating on atleast a portion of the interior surfaces of said gas passageway which isnon- Wettable by and resistant to chemical attack in use by the liquidatomic beam material, whereby creepage of liquid atomic beam materialthrough said gas passageway is prevented.

2. The apparatus of claim 1 wherein the atomic material is selected fromthe class consisting of cesium, thallium, sodium, rubidium, andpotassium.

3. The apparatus of claim 1 wherein said coated gas passageway portionsinclude a beam collimator structure.

4. The apparatus of claim 1 wherein said coated gas passageway portionsinclude an antispill tubulation projecting into said reservoir means.

5. The apparatus of claim 1 wherein said coated gas passageway has asubsurface formed of stainless steel having an oxidized Cr Oconstituent, and said dichlorodimethylsilane coating material ischemically bound to the gas passageway by a bond between oxygen atoms ofsaid oxidized Cr O constituent of the stainless steel and the siliconatoms of the coating material.

6. The apparatus of claim 1 including in combination, a collimator meansdefining the coated portion of said gas defining passageway forcollimating the atomic vapor into a collimated beam, means disposedalong the beam path for deflecting out of the beam certain atoms havingcertain energy states and retaining in the beam atoms having otherenergy states, means disposed along the beam path downstream of saidenergy selecting means for producing resonance of the beam particles,and means disposed at the terminal end of the beam path for detectingresonance of the beam particles to produce an output resonance signal.

References Cited UNITED STATES PATENTS WILLIAM F. LINDQUIST, PrimaryExaminer.

